Hi Daniel, yeah it’s hard.
I practiced more on a G and a low F to get it to hit right.
Finding the right tongue positioning is the key. Depending on the harp it is a matter of fine movements forward and back, near the teeth, to find it, plus variations in breath force.
I’ll be honest though, I rarely play up the top, at least not as much as I should.
That was probably zero help! Good luck.